to me, 9 weeks seems like a really long time to work on one project, especially with block schedules - how many hours is that? maybe for seniors, portfolio or independent projects it wouldn't be as bad...
most of our hs projects last from 2 - 4 weeks. each project has several graded components which are completed as the project progresses; students don't just get a grade on the final piece, their grade is an accumulation of points from different stages. also, 50% of each project grade is participation - little or no work in class = lost participation points for that day.
lastly, while i do think its important to have high expectations and to stick to them, i also believe there is nothing wrong with radically reworking or even ditching a project which seems to be widely unsuccessful. sometimes you can figure out what went wrong and make it better; sometimes it remains a mystery. finding projects that students like in my experience usually results in a high success rate. i don't mean you should assign "easy" projects; on the contrary, my students have really sunk their teeth into some of the more challenging subjects and media like an enlarged stippled ink drawing of nonobjective paper sculptures they built or scratchboard insect portraits.
if you were a terrible teacher, you wouldn't be asking these questions. you are obviously a caring, dedicated teacher to be so concerned!